1. Field of the Invention
This invention relates to methods, apparatus, and computer program code for driving an active matrix display, in particular an organic light emitting diode (OLED) display, with reduced power consumption.
2. Related Technology
Displays fabricated using OLEDs provide a number of advantages over LCD and other flat panel technologies. They are bright, colorful, fast-switching (compared to LCDs), provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colors which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixellated display. A multicolored display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix (AM) displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel while passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Examples of polymer and small-molecule active matrix display drivers can be found in WO 99/42983 and EP 0,717,446A respectively.
FIG. 1a shows such an example OLED active matrix pixel circuit 150. A circuit 150 is provided for each pixel of the display and ground 152, Vss 154, row select 124 and column data 126 busbars are provided interconnecting the pixels. Thus each pixel has a power and ground connection and each row of pixels has a common row select line 124 and each column of pixels has a common data line 126.
Each pixel has an organic LED 152 connected in series with a driver transistor 158 between ground and power lines 152 and 154. A gate connection 159 of driver transistor 158 is coupled to a storage capacitor 120 and a control transistor 122 couples gate 159 to column data line 126 under control of row select line 124. Transistor 122 is a thin film field effect transistor (FET) switch which connects column data line 126 to gate 159 and capacitor 120 when row select line 124 is activated. Thus when switch 122 is on a voltage on column data line 126 can be stored on a capacitor 120. This voltage is retained on the capacitor for at least the frame refresh period because of the relatively high impedances of the gate connection to driver transistor 158 and of switch transistor 122 in its “off” state.
Driver transistor 158 is typically an FET transistor and passes a (drain-source) current which is dependent upon the transistor's gate voltage less a threshold voltage. Thus the voltage at gate node 159 controls the current through OLED 152 and hence the brightness of the OLED.
The voltage-controlled circuit of FIG. 1 suffers from a number of drawbacks, and some ways to address these are described in the applicant's WO03/038790.
FIG. 1b, taken from WO03/038790, shows an example of a current-controlled pixel driver circuit 160 which addresses these problems. In this circuit the current through an OLED 152 is set by setting a drain source current for OLED driver transistor 158 using a reference current sink 162 and memorising the driver transistor gate voltage required for this drain-source current. Thus the brightness of OLED 152 is determined by the current, Icol, flowing into reference current sink 162, which is preferably adjustable and set as desired for the pixel being addressed. In addition, a further switching transistor 164 is connected between drive transistor 158 and OLED 152. In general one current sink 162 is provided for each column data line.
It can be seen from these examples that an active matrix pixel circuit generally incorporates a thin film (driver) transistor (TFT) in series with an electroluminescent display element.
Referring now to FIG. 2a, this shows drain characteristics 200 for a FET TFT driver transistor of an active matrix pixel circuit. A set of curves 202, 204, 206, 208, is shown each illustrating the variation of drain current of the FET with drain-source voltage for a particular gate-source voltage. After an initial non-linear portion of the curves become substantially flat, and the FET operates in the so-called saturation region. With increasing gate-source voltage the saturation drain current increases; below a threshold gate-source voltage VT the drain current is substantially 0. The dashed line 230 indicates the separation between the initial non-linear portion of the curves and the saturation region. For each set of curves 202, 204, 206, 208, a threshold point VT(202), VT(204), VT(206), VT(208) exists that indicates the point between the initial non-linear portion of the curve and the saturation region. Typical values of VT are between 1V and 6V. Broadly speaking the FET acts as a voltage controlled current limiter.
FIG. 2b shows a drive portion 240 of a typical active matrix pixel circuit. A PMOS driver FET 242 is connected in series with an organic light emitting diode 244 between a ground line 248 and a negative power line Vss 246.
It will be appreciated from the circuit of FIG. 2b that, for a given OLED drive current, the greater VSS the greater the excess (waste) power dissipation in driver transistor 242. It is therefore preferable to reduce VSS as much as possible to reduce this excess dissipated power. However it can be appreciated from FIG. 2a that there is a limit, as indicated by dashed line 230, below which Vss may not be reduced, this limit being determined by the maximum available VGS and the required OLED drive voltage.
In an active matrix driver multiple factors contribute to increasing the supply voltage of an AM OLED display above that which is necessary at a given time. In principal a supply voltage might only need to be ˜0.5V above that required to drive the highest voltage OLED (˜4V for polymer, ˜7V for small molecule and phosphorescent systems). However in practice the supply needs to be sufficient to hold the drive TFT in saturation, and possess enough overhead to cope with increases in OLED threshold voltage with time which can result in supply voltages as high as 14V for small molecule. This extra voltage is dropped entirely over the drive TFT increasing (doubling in the example given) the power consumption and stressing the TFT both with the enhanced field drop and heating. We have previously described, in WO03/107313, some techniques for addressing these difficulties.